Silicon carbide (SiC) has advantageous physical properties such as a wide band gap, a high thermal conductivity, and a low dielectric constant, which make it possible to realize semiconductor devices having a lower operational loss and a high resistance to temperature than silicon (Si) semiconductor devices. As a result, it is expected that silicon carbide can be used in a wide range of applications including as a material for power devices for electric power control, as a material for high frequency devices having high breakdown voltages, as a material for environmentally resistant devices for use in high temperature environments, as a material for radiation-resistant devices, and the like.
Each of these applications requires a high quality SiC single crystal and particularly a single crystal substrate having a diameter of at least 2 inches, and more specifically a SiC epitaxial wafer which comprises a SiC single crystal substrate having thereon an epitaxially grown SiC single crystal thin film which forms an active layer of a device.
The liquid phase epitaxy (LPE) method is known to form a SiC single crystal thin film having good crystalline quality. In the LPE method, first, carbon is dissolved in a melt of Si or a Si alloy to prepare a SiC solution having C (carbon) dissolved in the melt which serves as a solvent to a concentration close to its saturation point. A substrate as a seed crystal such as a substrate of a SiC single crystal is immersed in this SiC solution, and a supersaturated state of SiC is formed in the solution by creating a supercooled condition of the solution at least in the vicinity of the substrate, thereby causing epitaxial growth of a SiC single crystal on the substrate. This method can also be used to grow a SiC bulk single crystal. In this case, the crystal growth method is referred to as the solution growth method. Below, the LPE method and the solution growth method will collectively be referred to as the liquid phase growth method.
The LPE method, which is a liquid phase growth method, can have a higher film forming speed compared to the CVD method, which is a vapor phase growth method which is also capable of growth of a high quality SiC single crystal. In addition, in contrast to the CVD method which usually uses a so-called off-axis substrate in which the c axis is inclined, the LPE method has the advantage that the crystal orientation of the substrate is not limited so that it is possible to use a so-called on-axis substrate in which the c axis is not inclined. Use of an off-axis substrate is disadvantageous since it is known to have a tendency to easily introduce defects such as lattice defects into the single crystal being grown.
When application to an electronic device is contemplated, it is necessary to adjust the electrical conductivity of a SiC single crystal by controlling the carrier density thereof. For example, when a SiC single crystal thin film formed on a substrate is used as an active layer of a power device such as a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor), it is necessary to control the carrier density in the SiC single crystal to a desired value within the range of 1015/cm3 to 1016/cm3. When a SiC bulk single crystal is used as a substrate of a light emitting element (a thin film of GaN or the like is grown on the substrate), it is desirable to make the carrier density on the order of 1017/cm3 and to make the crystal transparent so that light can be emitted from its rear surface.
In the case of film formation of an active layer of a SiC single crystal by a vapor phase growth method such as the CVD method, carrier control is relatively easy since a SiC single crystal is grown under a reduced pressure in a glass vessel using high purity raw material gases.
However, growth of a SiC single crystal by the liquid phase growth method which uses a melt is usually carried out at atmospheric pressure in order to minimize vaporization of the melt, thereby making it difficult to control the carrier density of a SiC single crystal with good reproducibility. This is because in a SiC single crystal obtained by the liquid phase growth method, nitrogen gas which remains in a crystal growth chamber due to adsorption by a heat insulating material or the like unavoidably penetrates into the SiC single crystal being grown and replaces carbon sites of the crystal. As a result, the concentration of nitrogen atoms in the grown SiC single crystal reaches a level from 1018/cm3 to 1019/cm3. Furthermore, the resulting single crystal is strongly colored, and in general, it has a deep green color.
In a SiC semiconductor, nitrogen acts as a carrier in the form of an electron donor. Therefore, the carrier density increases as the nitrogen concentration increases. Accordingly, the above nitrogen concentration level indicates that the carrier density greatly exceeds the above-described preferred range. As a result, a SiC single crystal thin film grown by the LPE method and a SiC single crystal obtained by the solution growth method both have too high a carrier density, and they always exhibit a low specific resistivity on the order of several tens of milliohm-centimeter.
As described above, a SiC single crystal which is obtained by liquid phase growth is colored since it has a high carrier density due to incorporation of a large amount of nitrogen. Therefore, when a SiC single crystal obtained by the solution growth method is used as a substrate for a light emitting element, there is another problem in that it is difficult to discharge light from the rear surface of a substrate by the so-called flip chip mounting technique, and the SiC single crystal cannot be used as a substrate for lighting unit.
In addition, when a SiC thin film is used as an active layer of a power device such as a MOSFET, the SiC film becomes unsuitable for use as an active layer when the carrier density reaches a high level of 1018/cm3 or greater. Therefore, it is necessary for the SiC thin film deposited by the LPE method to again deposit thereon a SiC active layer having a thickness of several micrometers and a controlled carrier density by the CVD method or the like.
In this manner, with conventional liquid phase growth techniques for growing a SiC single crystal, since incorporation of nitrogen cannot be controlled, it was not possible to grow a SiC single crystal film having a suitable carrier density for an active layer of a power device on a substrate. In addition, because coloration was unavoidable, a bulk SiC single crystal manufactured for use as substrate could only be used in applications as a colored electrically conducting substrate, which made its uses extremely limited.
In below-identified Non-Patent Document 1, it is reported that growth of a SiC thin film with a low carrier density of 2×1016/cm3 was realized by growing a SiC single crystal by the LPE method (using molten Si as a solvent) in a graphite crucible under a reduced pressure of 5×10−4 Pa. However, in liquid phase growth under a reduced atmosphere, because the melt which acts as a solvent evaporates in a short period of time, it is not possible to stably carry out crystal growth over a prolonged period.
In below-identified Non-Patent Document 2, it is reported that a low carrier density of 8×1015/cm3 could be achieved by allowing a SiC solution in a solvent of a Si melt to heave by application of electromagnetic force so that the contact of the solution with the vessel wall is prevented and performing growth of a SiC single crystal in that condition under a reduced pressure of 5×10−5 torr. However, even in that method, because crystal growth is carried out under a reduced pressure, as described above, vaporization of the melt is unavoidable. Furthermore, because the melt is held in a heaved state by application of electromagnetic force, there is a limit to the weight of the melt that can be used, and it is difficult to grow a SiC single crystal film on a large substrate with a diameter of at least 2 inches.
Below-identified Patent Document 1 proposes a method of manufacturing a SiC single crystal by the LPE method using a SiC solution formed from a melt consisting of Si, C, and at least one transition metal (the solvent is a Si-transition metal alloy). That method is intended to enable the manufacture of a bulk single crystal (self-supporting crystals) of SiC with an increased growth speed, and there is no description concerning decreasing the carrier density or suppressing coloration of a SiC single crystal. Furthermore, the only actual examples of transition metals which are used in the examples are Mo, Cr, or Co used individually (namely, the solvent is a Si—Mo, Si—Cr, or Si—Co alloy).